Induction motors are perhaps the most widespread of all electric motors. Such motors require alternating current and may be found in both single phase and three phase power applications. Induction motors generally operate quite efficiently when fully loaded. Unfortunately, this efficiency drops when operating under less than full load conditions. When less than fully loaded, induction motors will consume more power than would otherwise be necessary to sustain operability at the given load.
Because of this, an effort has been made to provide motor voltage control devices that will reduce the amount of unneeded power delivered to induction motors when not fully loaded. These devices operate by monitoring the phase angle between the voltage and current wave forms in the motor, and by then reducing or raising the average voltage delivered to the motor in response to those observations.
U.S. Pat. No. 4,052,648 represents such a device. Briefly summarized, that device senses both the voltage and current flowing through the motor and produces square wave signals proportional thereto. These signals are logically combined and integrated to provide an error voltage signal. This error voltage signal may be influenced by the operator, who seeks to have the device maintain a particular pre-selected phase difference between the motor voltage and current. When the phase difference is other than this pre-selected difference, the device will detect this by comparing the error voltage signal with an internally generated saw-tooth signal and will then alter the delivery of voltage to the motor by controlling a triac in the power line.
By controlling this triac, the motor voltage wave form will be symmetrically chopped; that is, the motor voltage will be turned off for equal periods of time during both the positive and negative cycle of the voltage wave form. Therefore, the average voltage delivered to the motor will be reduced, and the total power consumed by the motor will be less. As the motor load decreases further, the device will turn the motor voltage off for a greater portion of each cycle. By the same token, as the motor load increases, the motor voltage will be turned on more frequently.
Certain problems remain or are created by such a device. In particular, with respect to the use of such a device in a single-phase application, the prior art has taught that motor misstarting may be avoided by delaying the activation of the motor voltage controller unit. In fact, while perhaps preventing the device itself from interferring with starting the motor, this does little to improve normal starting characteristics of the motor. Secondly, the large inrush currents associated with normal motor starting may cause undesirable arcing, light flicker and mechanical stress on the motor and associated equipment.
More seriously, the control device may fail and yet appear to continue to control the motor. The failure mode could include the presence of nonsymmetrical DC voltages in the motor, thereby giving rise to DC currents that could cause failure and even damage to the motor.
While a three-phase version of the device also suffers from the above shortcomings, an even greater problem becomes apparent. Specifically, the prior art teaches that the three-phase device should be restricted to use with three-phase motors having four leads, the fourth lead being connected to an internal ground between the phases. Most three-phase induction motors now in use, however, do not have such an accessible internal ground. To implement the use of such a device, then, would require the operator to dismantle such a motor and install a fourth lead to ground. The disadvantages of this requirement are obvious.
There exists, therefore, a need for a single-phase motor voltage control device having fail detect and soft start protection, and for a three-phase motor voltage control device having fail detect and soft start protection and further being generally usable with three-phase induction motors that do not have an accessible internal ground lead between phases.